scholarly journals Influence of Geometric Properties of Capacitive Sensors on Slope Error and Nonlinearity of Displacement Measurements

Sensors ◽  
2021 ◽  
Vol 21 (13) ◽  
pp. 4270
Author(s):  
Lars Daul ◽  
Tao Jin ◽  
Ingo Busch ◽  
Ludger Koenders

Capacitive sensors are widely used in industrial applications, such as CNC machine tools, where reliable positioning in the micrometer range with nanometer accuracy is required. Hence, these sensors are operated in harsh industrial environments. The accuracy of these sensors is mainly limited by slope errors and nonlinearities. In practice, the required accuracy of these sensors is achieved by a calibration against a metrological high-quality reference such as interferometric displacement measurement systems. This usually involves the use of high-order polynomials as calibration functions based on empirical data. In metrology, this is only the second-best approach and has disadvantages in terms of stability over the measurement range of the instrument. In addition, the validity of these empirical calibrations over time is questionable, and the associated uncertainty can only be roughly estimated. This makes regular recalibration of such sensors at short intervals mandatory to ensure the reliability of the displacement measurement. In this paper, we report on our investigations of the different parameters that affect the accuracy of capacitive sensors. Since the capacitance of these sensors results from the electric fields that build up between the electrodes, these field lines are calculated using FEM simulation models for typical commercial sensors. In the following the influence of various geometric parameters such as edge radius, guard ring size and shape, or thickness of the electrodes are individually analyzed according to their impact on the accuracy of these sensors. Based on these simulations, the deviations of the capacitance as they arise for real detector geometries can then be compared with idealized, de facto unrealizable parallel plate capacitors. This methodology allows overall uncertainty of capacitive sensors to be decomposed into their individual components and sorted in terms of their contribution to the uncertainty budget. The individual FEM-based analysis then enables a systematic analysis of the sources of uncertainty and, thus, reveals possibilities to improve manufacturing processes for capacitive sensors, to put these sensors on a solid metrological basis, and to improve the performance of these displacement measurement systems in the long run, i.e., to provide better sensors for the application.

Author(s):  
Michał Kowal ◽  
Roman Staniek

Accurate ballscrews are vital components of precise machine tool drive systems. As determined by direct measurement systems, the ballscrew positioning error has no bearing on the final positioning accuracy of the axis. For economical reasons, however, most machine tools are equipped with indirect measurement systems, in which errors stemming from thermal expansion of the ballscrew constitute approximately 60% of the kinematic chain error sum. Moreover, the currently observed boost in productivity of modern CNC machine tools leads to significant amplification of energy dispersal values in the nut-screw systems, due to the increased positioning velocity of the controlled axes. This, in turn, contributes to a rise of positioning error values through thermal expansion of the aforementioned ballscrews. This article deals with technological and constructional problems of screw lengthening compensation. It enumerates methods of thermal expansion-based error compensation as attained through utilization of indirect measurement systems. Finally, it presents experimental data indicating the possibility of effective screw lengthening compensation, thus proposing an alternative to the currently applied compensation systems.


2011 ◽  
Vol 141 ◽  
pp. 203-207
Author(s):  
Ya Wei Zhang ◽  
Wei Min Zhang

CNC machine tools has always screw joints in its feed drive systems; In order to obtain good performance of CNC machine tool, it is necessary to model the screw joint with more accuracy and to research its influence on the vibration characters of the feed drive system. In this paper, the screw joint is analyzed by multi-body system theory and is modeled as flexible multi-body; Its mathematical describe of constraint condition is given by the modeling of screw joint. A revise factor is introduced into the process of FEM simulation to reflect the deformation in the screw joint. By this way, the effect of deformation in the screw joint is researched in the modeling under the ANSYS circumstance, the harmonic response under considering deformation contrast to that of without deformation. From the analysis in the simulation, it is necessary to take the deformation of screw joint into account.


Sensors ◽  
2021 ◽  
Vol 21 (4) ◽  
pp. 1301
Author(s):  
Federico Cavedo ◽  
Parisa Esmaili ◽  
Michele Norgia

A low-cost optical reflectivity sensor is proposed in this paper, able to detect the presence of objects or surface optical properties variations, at a distance of up to 20 m. A collimated laser beam is pulsed at 10 kHz, and a synchronous digital detector coherently measures the back-diffused light collected through a 1-inch biconvex lens. The sensor is a cost-effective solution for punctual measurement of the surface reflection at different distances. To enhance the interference immunity, an algorithm based on a double-side digital baseline restorer is proposed and implemented to accurately detect the amplitude of the reflected light. As results show, the sensor is robust against ambient light and shows a strong sensitivity on a wide reflection range. The capability of the proposed sensor was evaluated experimentally for object detection and recognition, in addition to dedicated measurement systems, like remote encoders or keyphasors, realized far from the object to be measured.


Sign in / Sign up

Export Citation Format

Share Document